Pitch: How Fast Air Vibrates

In the last lesson we made one fixed tone. The thing that makes one tone different from another - high versus low, squeaky versus rumbly - is frequency.

Frequency = how many times the wave completes a full cycle in one second. One cycle = one hill and one valley. We measure it in Hertz (Hz). One Hertz means one cycle per second.

Machine 2 - Pitch Explorer

261.63

Freq 261.63 Hz

Oct 4

Volume 80%

What frequency looks like

Drag the slider. Watch the oscilloscope. As frequency goes up, the wave gets squished together - more cycles crammed into the same space. As frequency goes down, the wave stretches out lazily. Same shape, different speed.

Low pitch

High pitch

Both waves have the same amplitude (same volume). The only difference is how many cycles fit in the same time span. More cycles = higher pitch.

Try this: set the frequency below 80 Hz and watch the oscilloscope carefully. You can almost see individual cycles. Your speakers might also start physically moving in a visible way - which is either fascinating or alarming depending on your relationship with your speakers.

The range of human hearing

Human hearing runs from roughly 20 Hz (a bass so low it is more felt than heard) to 20,000 Hz (a piercing whistle that fades as you age). Most adults cannot hear above 15,000 Hz.

This machine goes up to 10,000 Hz. Above that, sine waves just become irritating.

Octaves

Try the octave buttons. An octave is what happens when you double the frequency. C4 is 261.63 Hz. C5 is 523.25 Hz - exactly double.

Your brain perceives doubled frequencies as "the same note, but higher." This is not a cultural convention. It is baked into auditory neuroscience across every human culture ever studied, and several animal species too.

A440 and the note system

The note buttons give you C4 through C5, plus A4 at 440 Hz - the reference pitch that orchestras tune to. When someone says "concert pitch" or "A440," they mean this exact frequency.

It was standardised in 1955 by the International Organisation for Standardisation, which is exactly the sort of committee you would expect to have opinions about the precise frequency of a musical note.

The frequency slider is logarithmic, not linear. Each equal distance corresponds to the same ratio of frequency change, not the same amount. This is because pitch perception is logarithmic - the jump from 100 Hz to 200 Hz sounds the same size as 1000 Hz to 2000 Hz. Both are one octave. If the slider were linear, the bottom 90% would all sound like bass.

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